The present invention relates to a fluid reservoir for mounting on a brake master cylinder, and more particularly to a fluid reservoir of the type which includes a diaphragm seal member coupled within a cap assembly of the reservoir casing to close off brake fluid stored in the casing from the surrounding air.
In such a conventional fluid reservoir as described above, the diaphragm seal member is expanded upward or downward in accordance with inflation or deflation of the air in the closed space to maintain the pressure in the closed space substantially at the atmospheric pressure. If the pressure in the closed space becomes positive or negative under maximum extension of the diaphragm seal member, unexpected troubles will occur in the vehicle braking system. To solve the problems, a conventional diaphragm seal member is provided thereon with inlet and outlet check valves which act to permit the flow of air to and from the closed space. It is, however, difficult to make the performance of the fluid reservoir uniform by provision of the inlet and outlet check valves.